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                <h2 id="线程与进程"><a href="#线程与进程" class="headerlink" title="线程与进程"></a>线程与进程</h2><p>  进程是程序的一次执行过程，是系统运行程序的基本单位，因此进程是动态的。系统运行一个程序即是一个进程从创建，运行到消亡的过程。<br>  线程与进程相似，但线程是一个比进程更小的执行单位。一个进程在其执行的过程中可以产生多个线程。与进程不同的是同类的多个线程共享进程的堆和方法区资源，但每个线程有自己的程序计数器、虚拟机栈和本地方法栈，所以系统在产生一个线程，或是在各个线程之间作切换工作时，负担要比进程小得多，也正因为如此，线程也被称为轻量级进程。<br>  <img src="https://img-blog.csdnimg.cn/65ae9c67bfbc44629a68861ee23b89d3.png" alt="在这里插入图片描述"><br>总结： 线程是进程划分成的更小的运行单位。线程和进程最大的不同在于基本上各进程是独立的，而各线程则不一定，因为同一进程中的线程极有可能会相互影响。线程执行开销小，但不利于资源的管理和保护；而进程正相反。</p>
<h2 id="同步和异步的区别"><a href="#同步和异步的区别" class="headerlink" title="同步和异步的区别"></a>同步和异步的区别</h2><p>同步 ： 发出一个调用之后，在没有得到结果之前， 该调用就不可以返回，一直等待。<br>异步 ：调用在发出之后，不用等待返回结果，该调用直接返回。</p>
<h2 id="上下文切换"><a href="#上下文切换" class="headerlink" title="上下文切换"></a>上下文切换</h2><p>线程在执行过程中会有自己的运行条件和状态（也称上下文），比如上文所说到过的程序计数器，栈信息等。当出现如下情况的时候，线程会从占用 CPU 状态中退出。</p>
<p>1.主动让出 CPU，比如调用了 sleep(), wait() 等。<br>2.时间片用完，因为操作系统要防止一个线程或者进程长时间占用 CPU 导致其他线程或者进程饿死。<br>3.调用了阻塞类型的系统中断，比如请求 IO，线程被阻塞。<br>4.被终止或结束运行</p>
<p>这其中前三种都会发生线程切换，线程切换意味着需要保存当前线程的上下文，留待线程下次占用 CPU 的时候恢复现场。并加载下一个将要占用 CPU 的线程上下文。这就是所谓的 上下文切换。</p>
<h2 id="sleep-方法和-wait-方法对比"><a href="#sleep-方法和-wait-方法对比" class="headerlink" title="sleep() 方法和 wait() 方法对比"></a>sleep() 方法和 wait() 方法对比</h2><p><strong>共同点</strong> ：</p>
<p>两者都可以暂停线程的执行。</p>
<p><strong>区别 ：</strong></p>
<p>sleep() 方法没有释放锁，而 wait() 方法释放了锁 。<br>wait() 通常被用于线程间交互&#x2F;通信，sleep()通常被用于暂停执行。<br>wait() 方法被调用后，线程不会自动苏醒，需要别的线程调用同一个对象上的 notify()或者 notifyAll() 方法。sleep()方法执行完成后，线程会自动苏醒，或者也可以使用 wait(long timeout) 超时后线程会自动苏醒。<br>sleep() 是 Thread 类的静态本地方法，wait() 则是 Object 类的本地方法。</p>
<h2 id="可以直接调用-Thread-类的-run-方法吗？"><a href="#可以直接调用-Thread-类的-run-方法吗？" class="headerlink" title="可以直接调用 Thread 类的 run 方法吗？"></a>可以直接调用 Thread 类的 run 方法吗？</h2><p>new 一个 Thread，线程进入了新建状态。调用 start()方法，会启动一个线程并使线程进入了就绪状态，当分配到时间片后就可以开始运行了。 start() 会执行线程的相应准备工作，然后自动执行 run() 方法的内容，这是真正的多线程工作。 但是，直接执行 run() 方法，会把 run() 方法当成一个 main 线程下的普通方法去执行，并不会在某个线程中执行它，所以这并不是多线程工作。</p>
<p>总结： 调用 start() 方法方可启动线程并使线程进入就绪状态，直接执行 run() 方法的话不会以多线程的方式执行。</p>
<h2 id="volatile-关键字"><a href="#volatile-关键字" class="headerlink" title="volatile 关键字"></a>volatile 关键字</h2><p>在 Java 中，volatile 关键字可以保证变量的可见性，如果我们将变量声明为 volatile ，这就指示 JVM，这个变量是共享且不稳定的，每次使用它都到主存中进行读取。<br>volatile 关键字能保证数据的可见性，但不能保证数据的原子性。synchronized 关键字两者都能保证。<br>在 Java 中，volatile 关键字除了可以保证变量的可见性，还有一个重要的作用就是防止 JVM 的指令重排序。 如果我们将变量声明为 volatile ，在对这个变量进行读写操作的时候，会通过插入特定的 内存屏障 的方式来禁止指令重排序。</p>
<h2 id="synchronized-关键字"><a href="#synchronized-关键字" class="headerlink" title="synchronized 关键字"></a>synchronized 关键字</h2><p>synchronized 关键字最主要的三种使用方式：</p>
<p>修饰实例方法<br>修饰静态方法<br>修饰代码块<br>1、修饰实例方法 （锁当前对象实例）</p>
<p>给当前对象实例加锁，进入同步代码前要获得 当前对象实例的锁 。</p>
<pre class=" language-java"><code class="language-java"><span class="token keyword">synchronized</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
    <span class="token comment" spellcheck="true">//业务代码</span>
<span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span>
</code></pre>
<p>2、修饰静态方法 （锁当前类）</p>
<p>给当前类加锁，会作用于类的所有对象实例 ，进入同步代码前要获得 当前 class 的锁。</p>
<p>这是因为静态成员不属于任何一个实例对象，归整个类所有，不依赖于类的特定实例，被类的所有实例共享。</p>
<pre class=" language-java"><code class="language-java"><span class="token keyword">synchronized</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
    <span class="token comment" spellcheck="true">//业务代码</span>
<span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span>
</code></pre>
<p>静态 synchronized 方法和非静态 synchronized 方法之间的调用互斥么？不互斥！如果一个线程 A 调用一个实例对象的非静态 synchronized 方法，而线程 B 需要调用这个实例对象所属类的静态 synchronized 方法，是允许的，不会发生互斥现象，因为访问静态 synchronized 方法占用的锁是当前类的锁，而访问非静态 synchronized 方法占用的锁是当前实例对象锁。</p>
<p>3、修饰代码块 （锁指定对象&#x2F;类）</p>
<p>对括号里指定的对象&#x2F;类加锁：</p>
<p>synchronized(object) 表示进入同步代码库前要获得 给定对象的锁。<br>synchronized(类.class) 表示进入同步代码前要获得 给定 Class 的锁</p>
<pre class=" language-java"><code class="language-java"><span class="token keyword">synchronized</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
    <span class="token comment" spellcheck="true">//业务代码</span>
<span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span>
</code></pre>
<p>总结：</p>
<p>synchronized 关键字加到 static 静态方法和 synchronized(class) 代码块上都是是给 Class 类上锁；<br>synchronized 关键字加到实例方法上是给对象实例上锁；<br>尽量不要使用 synchronized(String a) 因为 JVM 中，字符串常量池具有缓存功能。</p>
<h2 id="synchronized-和-volatile-的区别？"><a href="#synchronized-和-volatile-的区别？" class="headerlink" title="synchronized 和 volatile 的区别？"></a>synchronized 和 volatile 的区别？</h2><p>synchronized 关键字和 volatile 关键字是两个互补的存在，而不是对立的存在！</p>
<p>volatile 关键字是线程同步的轻量级实现，所以 volatile性能肯定比synchronized关键字要好 。但是 volatile 关键字只能用于变量而 synchronized 关键字可以修饰方法以及代码块 。<br>volatile 关键字能保证数据的可见性，但不能保证数据的原子性。synchronized 关键字两者都能保证。<br>volatile关键字主要用于解决变量在多个线程之间的可见性，而 synchronized 关键字解决的是多个线程之间访问资源的同步性。</p>
<h2 id="synchronized-和-ReentrantLock-的区别"><a href="#synchronized-和-ReentrantLock-的区别" class="headerlink" title="synchronized 和 ReentrantLock 的区别"></a>synchronized 和 ReentrantLock 的区别</h2><p><strong>两者都是可重入锁</strong><br>“可重入锁” 指的是自己可以再次获取自己的内部锁。比如一个线程获得了某个对象的锁，此时这个对象锁还没有释放，当其再次想要获取这个对象的锁的时候还是可以获取的，如果是不可重入锁的话，就会造成死锁。同一个线程每次获取锁，锁的计数器都自增 1，所以要等到锁的计数器下降为 0 时才能释放锁。</p>
<p><strong>synchronized 依赖于 JVM 而 ReentrantLock 依赖于 API</strong><br>synchronized 是依赖于 JVM 实现的，前面我们也讲到了 虚拟机团队在 JDK1.6 为 synchronized 关键字进行了很多优化，但是这些优化都是在虚拟机层面实现的，并没有直接暴露给我们。ReentrantLock 是 JDK 层面实现的（也就是 API 层面，需要 lock() 和 unlock() 方法配合 try&#x2F;finally 语句块来完成），所以我们可以通过查看它的源代码，来看它是如何实现的。</p>
<p><strong>ReentrantLock 比 synchronized 增加了一些高级功能</strong><br>相比synchronized，ReentrantLock增加了一些高级功能。主要来说主要有三点：</p>
<p><strong>等待可中断</strong> : ReentrantLock提供了一种能够中断等待锁的线程的机制，通过 lock.lockInterruptibly() 来实现这个机制。也就是说正在等待的线程可以选择放弃等待，改为处理其他事情。<br><strong>可实现公平锁</strong> : ReentrantLock可以指定是公平锁还是非公平锁。而synchronized只能是非公平锁。所谓的公平锁就是先等待的线程先获得锁。ReentrantLock默认情况是非公平的，可以通过 ReentrantLock类的ReentrantLock(boolean fair)构造方法来制定是否是公平的。<br><strong>可实现选择性通知</strong>（锁可以绑定多个条件）: synchronized关键字与wait()和notify()&#x2F;notifyAll()方法相结合可以实现等待&#x2F;通知机制。ReentrantLock类当然也可以实现，但是需要借助于Condition接口与newCondition()方法。<br> synchronized 适合锁少量的代码同步问题，Lock适合锁大量的同步代码。</p>
<h2 id="ThreadLocal（本地线程变量）"><a href="#ThreadLocal（本地线程变量）" class="headerlink" title="ThreadLocal（本地线程变量）"></a>ThreadLocal（本地线程变量）</h2><p>ThreadLocal 中填充的的是当前线程的变量，该变量对其他线程而言是封闭且隔离的，ThreadLocal 为变量在每个线程中创建了一个副本，这样每个线程都可以访问自己内部的副本变量。<br>每个Thread中都具备一个ThreadLocalMap，而ThreadLocalMap可以存储以ThreadLocal为 key ，Object 对象为 value 的键值对。<br>比如我们在同一个线程中声明了两个 ThreadLocal 对象的话， Thread内部都是使用仅有的那个ThreadLocalMap 存放数据的，ThreadLocalMap的 key 就是 ThreadLocal对象，value 就是 ThreadLocal 对象调用set方法设置的值。</p>
<p>ThreadLocal 数据结构如下图所示：<br><img src="https://img-blog.csdnimg.cn/ecc8a98caa7e4c6ca60da01a95daffac.png" alt="在这里插入图片描述"></p>
<h2 id="ThreadLocal-内存泄露问题"><a href="#ThreadLocal-内存泄露问题" class="headerlink" title="ThreadLocal 内存泄露问题"></a>ThreadLocal 内存泄露问题</h2><p>ThreadLocalMap 中使用的 key 为 ThreadLocal 的弱引用，而 value 是强引用。所以，如果 ThreadLocal 没有被外部强引用的情况下，在垃圾回收的时候，key 会被清理掉，而 value 不会被清理掉。<br>这样一来，ThreadLocalMap 中就会出现 key 为 null 的 Entry。假如我们不做任何措施的话，value 永远无法被 GC 回收，这个时候就可能会产生内存泄露。ThreadLocalMap 实现中已经考虑了这种情况，在调用 set()、get()、remove() 方法的时候，会清理掉 key 为 null 的记录。使用完 ThreadLocal方法后 最好手动调用remove()方法。</p>
<h2 id="线程池"><a href="#线程池" class="headerlink" title="线程池"></a>线程池</h2><p><strong>使用线程池的好处：</strong></p>
<p>降低资源消耗。通过重复利用已创建的线程降低线程创建和销毁造成的消耗。<br>提高响应速度。当任务到达时，任务可以不需要等到线程创建就能立即执行。<br>提高线程的可管理性。线程是稀缺资源，如果无限制的创建，不仅会消耗系统资源，还会降低系统的稳定性，使用线程池可以进行统一的分配，调优和监控。<br><strong>创建线程池</strong><br>《阿里巴巴 Java 开发手册》中强制线程池不允许使用 Executors 去创建，而是通过 ThreadPoolExecutor 的方式，这样的处理方式能更加明确线程池的运行规则，规避资源耗尽的风险。<br><strong>线程池三大方法</strong></p>
<pre class=" language-java"><code class="language-java">    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
        ExecutorService executorService <span class="token operator">=</span> Executors<span class="token punctuation">.</span><span class="token function">newSingleThreadExecutor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true">//单个线程</span>
<span class="token comment" spellcheck="true">//        ExecutorService executorService = Executors.newFixedThreadPool(5);//固定大小的</span>
<span class="token comment" spellcheck="true">//        ExecutorService executorService = Executors.newCachedThreadPool();//可伸缩</span>
        <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span><span class="token number">10</span><span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
            executorService<span class="token punctuation">.</span><span class="token function">execute</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">-</span><span class="token operator">></span><span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
                System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>Thread<span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"ok"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span>
        executorService<span class="token punctuation">.</span><span class="token function">shutdown</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span>
</code></pre>
<p><strong>线程池八大参数</strong></p>
<pre class=" language-java"><code class="language-java"> <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token operator">&amp;</span>#<span class="token number">123</span><span class="token punctuation">;</span>
        ExecutorService threadPool<span class="token operator">=</span><span class="token keyword">new</span> <span class="token class-name">ThreadPoolExecutor</span><span class="token punctuation">(</span>
                <span class="token number">2</span><span class="token punctuation">,</span><span class="token comment" spellcheck="true">//核心线程池大小为2</span>
                <span class="token number">5</span><span class="token punctuation">,</span><span class="token comment" spellcheck="true">//最大线程池大小为5</span>
                <span class="token number">3</span><span class="token punctuation">,</span><span class="token comment" spellcheck="true">//超时时间</span>
                TimeUnit<span class="token punctuation">.</span>SECONDS<span class="token punctuation">,</span>
                <span class="token keyword">new</span> <span class="token class-name">LinkedBlockingDeque</span><span class="token operator">&lt;</span><span class="token operator">></span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token comment" spellcheck="true">//阻塞队列</span>
                Executors<span class="token punctuation">.</span><span class="token function">defaultThreadFactory</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token comment" spellcheck="true">//线程工厂</span>
                <span class="token keyword">new</span> <span class="token class-name">ThreadPoolExecutor<span class="token punctuation">.</span>AbortPolicy</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment" spellcheck="true">//拒绝策略</span>
    <span class="token operator">&amp;</span>#<span class="token number">125</span><span class="token punctuation">;</span>
</code></pre>
<p><strong>四大拒绝策略</strong><br>ThreadPoolExecutor.AbortPolicy： 队列满了，还有进入，直接丢掉，抛出异常。<br>ThreadPoolExecutor.CallerRunsPolicy： 调用执行自己的线程运行任务，哪来的回哪里去。<br>ThreadPoolExecutor.DiscardPolicy： 队列满了，丢掉任务，不抛出异常。<br>ThreadPoolExecutor.DiscardOldestPolicy： 此策略将丢弃最早的未处理的任务请求。<br><strong>流程图</strong><br><img src="https://img-blog.csdnimg.cn/cfc70cde2b0643bf9cccf29e7408c076.png" alt="在这里插入图片描述"><br><strong>Atomic 原子类</strong></p>
<p>基本类型</p>
<p>AtomicInteger：整型原子类<br>AtomicLong：长整型原子类<br>AtomicBoolean：布尔型原子类</p>
<p>数组类型</p>
<p>AtomicIntegerArray：整型数组原子类<br>AtomicLongArray：长整型数组原子类<br>AtomicReferenceArray：引用类型数组原子类</p>
<p>引用类型</p>
<p>AtomicReference：引用类型原子类<br>AtomicStampedReference：原子更新带有版本号的引用类型。该类将整数值与引用关联起来，可用于解决原子的更新数据和数据的版本号，可以解决使用 CAS 进行原子更新时可能出现的 ABA 问题。<br>AtomicMarkableReference ：原子更新带有标记位的引用类型</p>
<pre class=" language-java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">get</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment" spellcheck="true">//获取当前的值</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">getAndSet</span><span class="token punctuation">(</span><span class="token keyword">int</span> newValue<span class="token punctuation">)</span><span class="token comment" spellcheck="true">//获取当前的值，并设置新的值</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">getAndIncrement</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token comment" spellcheck="true">//获取当前的值，并自增</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">getAndDecrement</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment" spellcheck="true">//获取当前的值，并自减</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">getAndAdd</span><span class="token punctuation">(</span><span class="token keyword">int</span> delta<span class="token punctuation">)</span> <span class="token comment" spellcheck="true">//获取当前的值，并加上预期的值</span>
<span class="token keyword">boolean</span> <span class="token function">compareAndSet</span><span class="token punctuation">(</span><span class="token keyword">int</span> expect<span class="token punctuation">,</span> <span class="token keyword">int</span> update<span class="token punctuation">)</span> <span class="token comment" spellcheck="true">//如果输入的数值等于预期值，则以原子方式将该值设置为输入值（update）</span>
<span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">lazySet</span><span class="token punctuation">(</span><span class="token keyword">int</span> newValue<span class="token punctuation">)</span><span class="token comment" spellcheck="true">//最终设置为newValue,使用 lazySet 设置之后可能导致其他线程在之后的一小段时间内还是可以读到旧的值。</span>
</code></pre>
<p>但是会出现aba问题</p>
<h2 id="AQS"><a href="#AQS" class="headerlink" title="AQS"></a>AQS</h2><p>AQS 是一个用来构建锁和同步器的框架，使用 AQS 能简单且高效地构造出大量应用广泛的同步器。<br>AQS 核心思想是，如果被请求的共享资源空闲，则将当前请求资源的线程设置为有效的工作线程，并且将共享资源设置为锁定状态。如果被请求的共享资源被占用，那么就需要一套线程阻塞等待以及被唤醒时锁分配的机制，这个机制 AQS 是用 CLH 队列锁实现的，即将暂时获取不到锁的线程加入到队列中。<br><img src="https://img-blog.csdnimg.cn/e87c64160a2946f2b96d01eb8884b5db.png" alt="在这里插入图片描述"><br>AQS 使用一个 int 成员变量来表示同步状态，通过内置的 FIFO 队列来完成获取资源线程的排队工作。AQS 使用 CAS 对该同步状态进行原子操作实现对其值的修改。</p>
<p>private volatile int state;&#x2F;&#x2F;共享变量，使用volatile修饰保证线程可见性<br>状态信息通过 protected 类型的 getState，setState，compareAndSetState 进行操作</p>
<p>&#x2F;&#x2F;返回同步状态的当前值<br>protected final int getState() {<br>    return state;<br>}<br>&#x2F;&#x2F;设置同步状态的值<br>protected final void setState(int newState) {<br>    state &#x3D; newState;<br>}<br>&#x2F;&#x2F;原子地（CAS操作）将同步状态值设置为给定值update如果当前同步状态的值等于expect（期望值）<br>protected final boolean compareAndSetState(int expect, int update) {<br>    return unsafe.compareAndSwapInt(this, stateOffset, expect, update);<br>}<br><strong>AQS 定义两种资源共享方式</strong></p>
<p>Exclusive（独占）：只有一个线程能执行，如 ReentrantLock。又可分为公平锁和非公平锁：<br>公平锁：按照线程在队列中的排队顺序，先到者先拿到锁<br>非公平锁：当线程要获取锁时，无视队列顺序直接去抢锁，谁抢到就是谁的<br>Share（共享）：多个线程可同时执行，如 CountDownLatch、Semaphore、 CyclicBarrier、ReadWriteLock 我们都会在后面讲到。<br>ReentrantReadWriteLock 可以看成是组合式，因为 ReentrantReadWriteLock 也就是读写锁允许多个线程同时对某一资源进行读。</p>
<p>不同的自定义同步器争用共享资源的方式也不同。自定义同步器在实现时只需要实现共享资源 state 的获取与释放方式即可，至于具体线程等待队列的维护（如获取资源失败入队&#x2F;唤醒出队等），AQS 已经在顶层实现好了。</p>

                
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